A need exists for a short-range radar system, which, for example, is suitable for automotive and other commercial applications. Such a system would be enabled to sense the proximity of other vehicles and objects, whether moving or stationary, within a range of about 0.15–30.0 meters.
U.S. Pat. No. 6,067,040 describes one conventional high resolution radar-based detection system. The system employs a ‘transmit’ channel which is connected to a first narrow pulse modulator 104, and emits pulse modulated transmit signals having a prescribed frequency and duration. The ‘receive’ channel is coupled to a second narrow pulse modulator 105 which assists in extracting the original transmit signals from the pulse modulated transmit signals reflected off an object (at range R) and received at the receive channel antenna. A time delay (τ) circuit delays the output of the second narrow pulse modulator 105 to the receive channel (so that the signal produced by the second narrow pulse modulator 105 is preferably in phase alignment with the reflected pulse modulated transmit signal received at the receive channel antenna), and a mixer 402 mixes the reflected pulse modulated transmit signals with the output of the second narrow pulse modulator 105 to produce the original (non-modulated) transmit signals.
In order to increase the range of a radar-based detection system (such as described above), the signal energy (power) of the transmitted signal must be increased. However, there are at least two limitations that restrict the amount of energy (power) that may be transmitted by a sensor system, such as the radar-based detection system described above. The first limitation is that the ability to discriminate between two targets (range discrimination) is a function of pulse length in pulsed radar systems. The second limitation is the ‘chirp’ or frequency modulation bandwidth in a continuous wave (CW) radar system. A longer pulse length increases the amount of energy transmitted by the sensor with a consequent reduction in the ability of the sensor to discriminate between closely located objects. Also, the interval between pulses may not be reduced indiscriminately to increase the transmitted energy (power) due to the need to maintain an unambiguous range measurement.
Additionally, a sensor system as described above is susceptible to in-band interference sources that produce electromagnetic (E-M) energy in same portion of the E-M spectrum as the sensor operates. The interfering sources include CW or pulsed transmissions by other systems, mutual interference from a second sensor or sensor system, self-jamming caused by imperfect isolation between the transmit and receive channels, and wide band thermal noise.
The use of such a sensor system in the automotive industry becomes particularly problematic due to sheer volume of automobiles which may occupy a particular space at any one time (e.g., highway). For example, hundreds of cars on a single highway may all be generating and receiving sensor signals in the same frequency range.
U.S. Pat. No. 6,587,072 describes a radar-based sensor system which eliminates some of the above-mentioned deficiencies. The system shown in FIG. 1 of the '072 Patent includes a ‘transmit’ arm including a signal source 10, a frequency correction module 11, a buffer amplifier 12, a pulse former 13, a resistive element 14, a switch driver 15, a bi-phase modulator 16, a modulator driver 17, output amplifiers 25 and 26, variable gain control 27, and a switch driver 28. A ‘receive’ arm of the system includes low noise amplifiers (LNAs) 40 and 41, phase shifter 43, mixers 44 and 45, integrator circuits 47 and 48, doppler filters 49 and 50, sampler circuit 51, switches 52 and 54, and integrator circuits 53 and 55.
A transmit-receive select switch 18 selects which arm (e.g., ‘transmit’ or ‘receive’) of the system is in operation at any particular time. When the ‘transmit’ arm is in operation, transmit antenna 30 sends signals toward an object 35. When the ‘receive’ arm is in operation, receive antenna 31 receives signals which are reflected back from the object 35. A benefit of the sensor system is that it may transmit pulses of differing pulse lengths to increase the range of the system. For example, if one pulse is defined as a ‘chip’, the pulse length may be increased to 3 chips, 7 chips, 11 chips, and so on, to accurately identify objects which are farther away. Additional specifics of the radar-based sensor system are described in detail in the '072 Patent, and are incorporated herein by reference.
However, the radar-based sensor system described in the '072 Patent requires an integrator which can accurately integrate over variable pulse widths (e.g., 1 chip, 3 chips, 5 chips, etc.).
Thus, there is presently a need for an integrator which can accurately and efficiently integrate pulses with various pulses widths.